Internal combustion engine with an exhaust-gas recirculation and method for operating an internal combustion engine

ABSTRACT

An internal combustion engine has a high-pressure exhaust-gas recirculation line and a low-pressure exhaust-gas recirculation line with a low-pressure exhaust-gas recirculation valve. The low-pressure exhaust-gas recirculation line branches off from the exhaust-gas system downstream of a turbine of an exhaust-gas turbocharger and opens into a fresh-air system upstream of a compressor of the exhaust-gas turbocharger. An exhaust-gas flap is disposed in the exhaust-gas system downstream from where the low-pressure exhaust-gas recirculation line branches off from the exhaust-gas system. At least one pressure sensor is disposed in the low-pressure exhaust-gas recirculation line and configured such that the at least one pressure sensor determines a pressure difference in the low-pressure exhaust-gas recirculation line between a point upstream of the low-pressure exhaust-gas recirculation valve and a point downstream of the low-pressure exhaust-gas recirculation valve. A method for operating an internal combustion engine is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S.application Ser. No. 12/466,708, filed May 15, 2009, which is acontinuation, under 35 U.S.C. §120, of copending InternationalApplication No. PCT/EP2007/008606, filed Oct. 4, 2007, which designatedthe United States; this application also claims the priority, under 35U.S.C. §119, of German Patent Application No. DE 10 2006 054 043.3,filed Nov. 16, 2006; the prior applications are herewith incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to an internal combustion engine, in particular adiesel engine or an otto-cycle engine, with a fresh-air system in whicha charge-air cooler is disposed, an exhaust-gas system, an exhaust-gasturbocharger which has a compressor disposed in the fresh-air systemupstream of the charge-air cooler and a turbine disposed in theexhaust-gas system, a first line for a high-pressure exhaust-gasrecirculation which, upstream of the turbine of the exhaust-gasturbocharger, branches off from the exhaust-gas system and which has ahigh-pressure exhaust-gas recirculation valve and which, downstream ofthe charge-air cooler, opens into the fresh-air system, a second linefor a low-pressure exhaust-gas recirculation which, downstream of theturbine of the exhaust-gas turbocharger, branches off from theexhaust-gas system and which has a low-pressure exhaust-gasrecirculation valve and which, upstream of the compressor of theexhaust-gas turbocharger, opens into the fresh-air system, and anexhaust-gas flap disposed in the exhaust-gas system downstream of thebranch-off of the low-pressure exhaust-gas recirculation line. Theinvention also relates to a method for operating an internal combustionengine, particularly a diesel engine or an otto-cycle engine, theinternal combustion engine having a fresh-air system, an exhaust-gassystem, an exhaust-gas turbocharger, which has a compressor disposed inthe fresh-air system and a turbine disposed in the exhaust-gas system,wherein recirculated exhaust-gas is diverted from the exhaust-gas systemdownstream of the turbine of the exhaust-gas turbocharger, via a linefor a low-pressure exhaust-gas recirculation, by using a low-pressureexhaust-gas recirculation valve and is fed, upstream of the compressorof the exhaust-gas turbocharger, as a low-pressure exhaust-gasrecirculation mass flow into the fresh-air system, wherein anexhaust-gas flap is disposed downstream of the branch-off of thelow-pressure exhaust-gas recirculation line.

Japanese Patent Application JP 2004150319 A discloses a dieselcombustion engine with a high-pressure exhaust-gas recirculation path(HP-EGR path) and a low-pressure exhaust-gas recirculation path (LP-EGRpath), wherein an exhaust-gas flap is provided at the junction where theLP-EGR path branches off from an exhaust-gas line of the dieselcombustion engine. The LP-EGR mass flow is extracted downstream of aturbine of an exhaust-gas turbocharger (EGT) on the low pressure sideafter a device for an exhaust-gas aftertreatment and is fed into theintake air in front of a compressor of the exhaust-gas turbocharger. Thelow-pressure exhaust-gas recirculation mass flow is cooled down on theway to the compressor inlet. A low-pressure exhaust-gas recirculationvalve is provided upstream of the entry of the low-pressure exhaust-gasrecirculation mass flow into the fresh air. The low-pressure exhaust-gasrecirculation path is used at low and medium rotational speeds and inthe middle load range, i.e. when there is a sufficient pressure drop.

Japanese Patent Application JP 2004156572 A discloses a diesel enginewhich has a high-pressure exhaust-gas recirculation path (HP-EGR path)with a high-pressure exhaust-gas recirculation valve and a low-pressureexhaust-gas recirculation path (LP-EGR path). The low-pressureexhaust-gas recirculation path branches off downstream of a turbine ofan exhaust-gas turbocharger (EGT) as well as after an exhaust-gasaftertreatment device and opens into the fresh-air path upstream of acompressor of the exhaust-gas turbocharger. An exhaust-gas flap controlsthe amount of the low-pressure exhaust-gas recirculation mass flow inaccordance with an operating state of the diesel engine.

U.S. Pat. No. 7,013,879 B2 discloses a low-pressure exhaust-gasrecirculation path (LP-EGR path) for an internal combustion engine,wherein the LP-EGR path branches off downstream of the turbine of anexhaust-gas turbocharger as well as a particulate filter and, via an EGRcooler as well as a control valve, opens into the fresh-air linedirectly upstream of the compressor of the exhaust-gas turbocharger.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an internalcombustion engine and a method for operating an internal combustionengine which have an improved exhaust-gas processing and exhaust-gasaftertreatment.

With the foregoing and other objects in view there is provided, inaccordance with the invention, an internal combustion engine, including:

a fresh-air system;

a charge-air cooler disposed in the fresh-air system;

an exhaust-gas system;

an exhaust-gas turbocharger having a compressor and a turbine, thecompressor being disposed, upstream of the charge-air cooler, in thefresh-air system, the turbine being disposed in the exhaust-gas system;

a high-pressure exhaust-gas recirculation line branching off from theexhaust-gas system upstream of the turbine of the exhaust-gasturbocharger, the high-pressure exhaust-gas recirculation line having ahigh-pressure exhaust-gas recirculation valve, the high-pressureexhaust-gas recirculation line opening into the fresh-air systemdownstream of the charge-air cooler;

a low-pressure exhaust-gas recirculation line branching off from theexhaust-gas system downstream of the turbine of the exhaust-gasturbocharger, the low-pressure exhaust-gas recirculation line having alow-pressure exhaust-gas recirculation valve, the low-pressureexhaust-gas recirculation line opening into the fresh-air systemupstream of the compressor of the exhaust-gas turbocharger;

an exhaust-gas flap disposed in the exhaust-gas system downstream fromwhere the low-pressure exhaust-gas recirculation line branches off fromthe exhaust-gas system; and

at least one pressure sensor disposed in the low-pressure exhaust-gasrecirculation line and configured such that the at least one pressuresensor determines a pressure difference in the low-pressure exhaust-gasrecirculation line between a point upstream of the low-pressureexhaust-gas recirculation valve and a point downstream of thelow-pressure exhaust-gas recirculation valve.

According to another feature of the invention, the at least one pressuresensor includes a differential pressure sensor. According to a furtherfeature of the invention, an exhaust-gas recirculation cooler isdisposed in the high-pressure exhaust-gas recirculation line. Accordingto yet another feature of the invention, a particulate filter isdisposed in the exhaust-gas system downstream of the turbine of theexhaust-gas turbocharger.

According to another feature of the invention, the low-pressureexhaust-gas recirculation line branches off from the exhaust-gas systemdownstream of the particulate filter. According to a further feature ofthe invention, an exhaust-gas recirculation cooler is disposed in thelow-pressure exhaust-gas recirculation line.

According to another feature of the invention, the at least one pressuresensor includes an absolute pressure sensor disposed in flow directionin front of the exhaust-gas recirculation cooler. According to anotherfeature of the invention, the exhaust-gas recirculation cooler isdisposed upstream of the low-pressure exhaust-gas recirculation valve.

According to another feature of the invention, a temperature sensor isdisposed in the low-pressure exhaust-gas recirculation line downstreamof the low-pressure exhaust-gas recirculation valve. According toanother feature of the invention, a temperature sensor is disposed inthe exhaust-gas system upstream of the turbine of the exhaust-gasturbocharger. According to a further feature of the invention, theinternal combustion engine is a diesel engine or an otto-cycle engine.

In other words, according to the invention, there is provided aninternal combustion engine, in particular a diesel engine or anotto-cycle engine, with a fresh-air system in which a charge-air cooleris disposed, an exhaust-gas system, an exhaust-gas turbocharger whichhas a compressor disposed in the fresh-air system upstream of thecharge-air cooler and a turbine disposed in the exhaust-gas system, afirst line for a high-pressure exhaust-gas recirculation which, upstreamof the turbine of the exhaust-gas turbocharger, branches off from theexhaust-gas system and which has a high-pressure exhaust-gasrecirculation valve and which, downstream of the charge-air cooler,opens into the fresh-air system, a second line for a low-pressureexhaust-gas recirculation which, downstream of the turbine of theexhaust-gas turbocharger, branches off from the exhaust-gas system andwhich has a low-pressure exhaust-gas recirculation valve and which,upstream of the compressor of the exhaust-gas turbocharger, opens intothe fresh-air system, and an exhaust-gas flap disposed in theexhaust-gas system downstream of the branch-off of the low-pressureexhaust-gas recirculation line, wherein, in the low-pressure exhaust-gasrecirculation line, at least one pressure sensor is disposed andconfigured such that it determines a pressure difference in thelow-pressure exhaust-gas recirculation line between a point upstream anda point downstream of the low-pressure exhaust-gas recirculation valve.

Providing the pressure sensor in the above-defined manner, has theadvantage that a real low-pressure exhaust-gas recirculation mass flowcan be determined, which can be used as a base value for controlling theexhaust-gas flap and the low-pressure exhaust-gas recirculation valve(LP-EGR valve). The pressure sensor includes for example a differentialpressure sensor.

An exhaust-gas recirculation cooler (EGR cooler) is expediently disposedin the high-pressure exhaust-gas recirculation line and a particulatefilter is expediently disposed in the exhaust-gas system downstream ofthe turbine of the exhaust-gas turbocharger, wherein the low-pressureexhaust-gas recirculation line branches off from the exhaust-gas systemat a point downstream of the particulate filter.

In order to protect the compressor from an excessive thermal load and inorder to be able to compress the mixture of exhaust-gas and fresh airsufficiently highly, an exhaust-gas recirculation cooler (EGR cooler) isdisposed in the low-pressure exhaust-gas recirculation line (LP-EGRline). The EGR cooler is preferably disposed upstream of the EGR valve.

In accordance with a preferred embodiment, the pressure sensor includesan absolute pressure sensor which is disposed in flow direction in frontof the EGR cooler. A temperature sensor is disposed downstream of theLP-EGR valve in the LP-EGR line for the purpose of monitoring possibleon-board diagnostic (OBD) requirements. A temperature sensor isexpediently disposed in the exhaust-gas system upstream of the turbine.

With the objects of the invention in view there is also provided, amethod for operating a combustion engine that includes the followingsteps:

providing an internal combustion engine including a fresh-air system, anexhaust-gas system, an exhaust-gas turbocharger with a compressordisposed in the fresh-air system and a turbine disposed in theexhaust-gas system and providing an exhaust-gas flap downstream fromwhere a low-pressure exhaust-gas recirculation line branches off fromthe exhaust-gas system;

diverting, via the low-pressure exhaust-gas recirculation line,recirculated exhaust-gas from the exhaust-gas system downstream of theturbine of the exhaust-gas turbocharger by using a low-pressureexhaust-gas recirculation valve and feeding recirculated exhaust-gasupstream of the compressor of the exhaust-gas turbocharger as alow-pressure exhaust-gas recirculation mass flow into the fresh-airsystem; and

opening the exhaust-gas flap and controlling a volume flow through thelow-pressure exhaust-gas recirculation line exclusively with thelow-pressure exhaust-gas recirculation valve as long as a pressure dropacross the low-pressure exhaust-gas recirculation valve exceeds a givenvalue and additionally actuating the exhaust-gas flap in a direction ofclosing in case the pressure drop across the low-pressure exhaust-gasrecirculation valve falls below the given value in order to control thevolume flow through the low-pressure exhaust-gas recirculation line.

Another mode of the method of the invention includes diverting therecirculated exhaust-gas from the exhaust-gas system downstream of aparticulate filter. A further mode of the method of the inventionincludes diverting, via a high-pressure exhaust-gas recirculation line,recirculated exhaust-gas from the exhaust-gas system upstream of theturbine of the exhaust-gas turbocharger by using a high-pressureexhaust-gas recirculation valve and feeding recirculated exhaust-gasdownstream of the compressor of the exhaust-gas turbocharger as ahigh-pressure exhaust-gas recirculation mass flow into the fresh-airsystem.

Another mode of the method of the invention includes feeding thehigh-pressure exhaust-gas recirculation mass flow downstream of acharge-air cooler into the fresh-air system. A further mode of themethod of the invention includes cooling the high-pressure exhaust-gasrecirculation mass flow prior to feeding the high-pressure exhaust-gasrecirculation mass flow into the fresh-air system.

Another mode of the method of the invention includes measuring apressure difference across the low-pressure exhaust-gas recirculationvalve; and actuating the low-pressure exhaust-gas recirculation valveand/or the exhaust-gas flap in dependence of the pressure differenceacross the low-pressure exhaust-gas recirculation valve. Another mode ofthe method of the invention includes measuring a temperature of thelow-pressure exhaust-gas recirculation mass flow downstream of thelow-pressure exhaust-gas recirculation valve.

Another mode of the method of the invention includes calculating alow-pressure exhaust-gas mass flow by using an ascertained differentialpressure across the low-pressure exhaust-gas recirculation valve; anddetermining, from the low-pressure exhaust-gas mass flow, a volume flowthrough a particulate filter disposed in the exhaust-gas system. Anothermode of the method of the invention includes determining, from thevolume flow through the particulate filter, a loading of the particulatefilter.

Another mode of the method of the invention includes cooling thelow-pressure exhaust-gas recirculation mass flow prior to feeding thelow-pressure exhaust-gas recirculation mass flow into the fresh-airsystem. Another mode of the method of the invention includes using, asthe internal combustion engine, a diesel engine or an otto-cycle engine.

In other words, according to the invention, there is provided a methodfor operating an internal combustion engine, particularly a dieselengine or an otto-cycle engine, which is in particular embodied inaccordance with at least one of the above-defined configurations, theinternal combustion engine having a fresh-air system, an exhaust-gassystem, an exhaust-gas turbocharger (EGT), which has a compressordisposed in the fresh-air system and a turbine disposed in theexhaust-gas system, wherein recirculated exhaust-gas is diverted fromthe exhaust-gas system downstream of the turbine of the exhaust-gasturbocharger, via a line for a low-pressure exhaust-gas recirculation(LP-EGR), by using a low-pressure exhaust-gas recirculation valve, andis fed, upstream of the compressor of the exhaust-gas turbocharger, as alow-pressure exhaust-gas recirculation mass flow into the fresh-airsystem, wherein an exhaust-gas flap is disposed downstream of thebranch-off of the low-pressure exhaust-gas recirculation line, whereinthe exhaust-gas flap is open and the volume flow via the LP-EGR line iscontrolled exclusively through the use of the low-pressure exhaust-gasrecirculation valve as long as a pressure drop across the low-pressureexhaust-gas recirculation valve exceeds a predetermined value andwherein, when it falls below the predetermined value, the exhaust-gasflap is actuated in addition in a closing direction in order to controlthe volume flow via the low-pressure exhaust-gas recirculation line.

The above-defined method has the advantage that a very good mixing ofthe recirculated exhaust-gas and the fresh air is achieved, wherein therecirculated exhaust-gas is practically non-pulsating. A particularlyclean recirculated exhaust-gas is achieved by diverting the recirculatedexhaust-gas downstream of a particulate filter.

A further reduction of pollutants in the exhaust-gas is achieved bydiverting recirculated exhaust-gas from the exhaust-gas system via aline for a high-pressure exhaust-gas recirculation (HP-EGR) with aHP-EGR valve upstream of the turbine of the exhaust-gas turbocharger andfeeding it downstream of the compressor of the exhaust-gas turbochargeras a high-pressure exhaust-gas recirculation (HP-EGR) mass flow into thefresh-air system.

The high-pressure exhaust-gas recirculation (HP-EGR) mass flow isexpediently fed into the fresh-air system downstream of a charge-aircooler. It is furthermore advantageous to cool the HP-EGR mass flowprior to feeding it into the fresh-air system. In order to accuratelycontrol the low-pressure exhaust-gas recirculation mass flow, a pressuredifference across the low-pressure exhaust-gas recirculation valve ismeasured and the low-pressure exhaust-gas recirculation valve and/or theexhaust-gas flap are actuated in dependence of this pressure difference.

A temperature of the low-pressure exhaust-gas recirculation mass flow ismeasured downstream of the low-pressure exhaust-gas recirculation valvein order to monitor possible on-board diagnostic requirements. It isexpedient to calculate the low-pressure exhaust-gas mass flow from anascertained differential pressure across the low-pressure exhaust-gasrecirculation valve and to determine a volume flow through a particulatefilter disposed in the exhaust-gas system from this low-pressureexhaust-gas mass flow. Furthermore a loading of the particulate filteris then determined from the volume flow through the particulate filter.In order to protect the compressor from excessive thermal stress and inorder to be able to compress the mixture of exhaust-gas and fresh airsufficiently highly, the low-pressure exhaust-gas recirculation massflow is cooled before introducing it into the fresh-air system.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an internal combustion engine and a method for operating an internalcombustion engine, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a schematic view of a preferred embodiment of aninternal combustion engine according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the single FIGURE, there is shown a preferredembodiment of an internal combustion engine according to the inventionin the form of a diesel combustion engine. The diesel combustion engineincludes an engine block 10 with working cylinders 12, wherein eachworking cylinder 12 has a cylinder pressure sensor 14 and a common railinjector 16 assigned thereto. A fresh-air system 18 supplies fresh airas combustion air to the working cylinders 12 and an exhaust-gas system20 discharges exhaust-gas from the working cylinders 12.

The following elements are provided in the fresh-air system 18, whenviewed in the flow direction of the fresh air 22: An air filter 24, anair flow meter 26 (HFM=hot-film air-mass meter), a compressor 28 of anexhaust-gas turbocharger (EGT) 30, a charge-air cooler 32, a duo-sensor34, which measures a temperature T₂ as well as a pressure p₂, a controlflap 36, and a controlled intake manifold runner 38.

The following elements are provided in the exhaust-gas system 20starting from the engine block 10, when viewed in the flow direction ofthe exhaust-gas 40: A first temperature sensor 42, which measures atemperature T₃, a turbine 44 of the exhaust-gas turbocharger 30, adisplacement sensor 46 disposed at the turbine 44, a first Lambda oxygensensor 48, a second temperature sensor 50, which measures a temperatureT₄, an oxidation catalytic converter (DOC=diesel oxidation catalyticconverter) 52, a third temperature sensor 54, which measures atemperature T*, a diesel particulate filter (DPF) 56, a firstdifferential pressure sensor 58, which determines a pressure drop acrossthe diesel particulate filter (DPF) 56, a fourth temperature sensor 60,which determines a temperature T₄′, an exhaust-gas flap 62, an NO_(x)storage catalytic converter 64, a second Lambda oxygen sensor 66 and anH₂S trap catalytic converter (OC) 68.

Downstream of the diesel particulate filter 56, i.e. on a low-pressureside of the exhaust-gas system 20, a low-pressure exhaust-gasrecirculation line (LP-EGR line) 70 branches off from the exhaust-gassystem 20, the LP-EGR line 70 merging back into the fresh-air system 18upstream of the compressor 28 and downstream of the air filter 24 or theair-mass meter (HFM) 26. In the low-pressure exhaust-gas recirculationline (LP-EGR line) 70, the following elements are provided, when viewedin the flow direction of an LP-EGR mass flow 72, starting at thebranch-off from the exhaust-gas system 20: An EGR cooler 74, a LP-EGRvalve 76, a second differential pressure sensor 78, which determines apressure drop across the EGR cooler 74 and the LP-EGR valve 76, a fifthtemperature sensor 80, which determines a temperature T_(LP EGR) of theLP-EGR mass flow 72.

The cooling of the LP-EGR mass flow can optionally be performed byomitting the EGR cooler 74 and utilizing the lengths of the lines orutilizing the geometries or shapes of the lines or pipes.

Upstream of the turbine 44 of the exhaust-gas turbocharger 30, i.e. on ahigh-pressure side of the exhaust-gas system 20, a high-pressureexhaust-gas recirculation line (HP-EGR line) 82 branches off from theexhaust-gas system 20, which high-pressure exhaust-gas recirculationline 82 opens into the fresh-air system 18 downstream of the controlflap 36 and upstream of the controlled intake manifold runner. Throughthe use of this high-pressure exhaust-gas recirculation line (HP-EGRline) 82, an HP-EGR mass flow 84 is fed, via an HP-EGR valve 86, intothe fresh-air system 18. An EGR cooler 88 is shown in the HP-EGR line 82of the embodiment shown in the FIGURE, however, in accordance with analternative embodiment, the EGR cooler 88 in the HP-EGR line 82 may beomitted. A cooling of the HP-EGR mass flow 84 can optionally be carriedout through the use of either the length of line of the HP-EGR line 82or through the use of the EGR cooler 88, which is provided in the HP-EGRline 82 and which can be structurally and/or functionally combined withthe EGR cooler 74 of the LP-EGR path.

The internal combustion engine shown in the figure therefore has anexhaust-gas recirculation (EGR) in which, upstream of the turbine 44 aswell as downstream of the diesel particulate filter 56, exhaust-gas isextracted from the exhaust-gas system 20 and, after being cooled, is fedinto the fresh-air system 18 upstream of the compressor 28 as well asdownstream of the charge-air cooler 32. The control of the EGR amount,i.e. the recirculated exhaust-gas mass flow, is carried out through theuse of the exhaust-gas flap 62 and the EGR valves 76 and 86. Theinternal combustion engine can alternatively be operated without anexhaust-gas recirculation, with a high-pressure exhaust-gasrecirculation (HP-EGR) or low-pressure exhaust-gas recirculation(LP-EGR) or simultaneously with a high-pressure exhaust-gasrecirculation (HP-EGR) and a low-pressure exhaust-gas recirculation(LP-EGR). Altogether, this achieves a clean EGR mass flow 72 and 84, animproved cooling of the EGR mass flow 72 and, respectively, 84, no sootaccumulation in the EGR cooler, a good mixing of the EGR mass flow 72and, respectively, 84 and the fresh air 88 in the fresh-air system. HighEGR rates are possible and a partly homogeneous operation of theinternal combustion engine is possible.

With the help of the exhaust-gas flap 62 and the LP-EGR valve 76 infront of the compressor 28, the recirculated LP-EGR mass flow 72 is fedto the fresh air 88 between the air-mass meter 26 and the compressor 28.In this case, the operation is initially performed only by the LP-EGRvalve 76, as long as a sufficient pressure drop, i.e. difference inpressure, is available for the delivery of the LP-EGR mass flow 72. Ifthis is no longer the case, the exhaust-gas flap 62 is adjusted toincrease its pitch to some extent, in order to increase the pressuredrop across the LP-EGR valve 76. A very good mixing of the LP-EGR massflow 72 with the fresh air 88 is ensured. A further advantage amongothers is that the recirculated exhaust-gas 72 is clean and almostnon-pulsating. In addition, an increased compressor capacity isavailable since during the LP-EGR operation the entire exhaust-gas 40 isconducted through the turbine and not, as is the case in a conventionalsetup only a partial flow which is not recirculated. Since therecirculated exhaust-gas 72 after the compressor 28 is conducted throughthe high-capacity charge-air cooler 32, the exhaust-gas/air mixture isalso relatively cold. The engine can be operated depending on need, bothwith the HP-EGR path 82 and with the LP-EGR path 70. A number of sensorsand actuators are needed for the operation of the engine whose functionsare explained hereinafter.

The EGR system of the internal combustion engine according to theinvention includes among others the following components:

1. Exhaust-gas flap 62 (electric, position control in the engine controlunit)

In the LP-EGR path 70:

2. EGR valve 76 (electric with position feedback, position control bythe engine control unit)

3. EGR cooler 74

4. Differential pressure sensor device 78, alternatively absolutepressure in front of the EGR cooler 74 (in flow direction)

5. EGR temperature sensor 80 T_(LP-EGR), optional for implementing theOBD (On-Board Diagnostics) requirements in NAR (North America Region)

The EGR cooler makes sure that no inadmissibly high temperatures occurat the compressor 28 during the EGR operation. The differential pressuresensor 78 serves for acquiring the exhaust-gas volume flow through theLP-EGR path 70. This volume flow is used for a volume flow calculationacross the diesel particulate filter 56 and thus for determining theloading. This volume flow furthermore serves as an actual quantity ofthe exhaust-gas rate for the exhaust-gas rate control. The temperaturesensor 80 has two tasks: It serves for diagnostic purposes (efficiencymonitoring of the cooler) as well as for switching off the EGR in caseof an imminent excess temperature of the compressor 28. The EGR cooler74 can be dimensioned such that it is smaller than it would be withoutthis temperature sensor 80.

The exhaust-gas flap 62 and the LP-EGR valve 76 are the actuatingelements of the EGR regulation. Both, the LP-EGR valve and theexhaust-gas flap can be adjusted in a continuous manner. In principle,one distinguishes between two different operating modes:

1. Normal Operation (Driving Operation with Exhaust-Gas Recirculation):

The fresh-air mass 88 is controlled in a usual manner. The actual valueof the fresh-air mass 88 is measured with the HFM 26 and is comparedwith a setpoint value in a control unit. The control flap 36 iscompletely open. The HP-EGR valve 86 is in this case controlled. Thecontrol variable (manipulated variable) of the controller operates onthe LP-EGR valve 76 in case of small control variables and theexhaust-gas flap 62 remains open for an optimal efficiency. Only if theLP-EGR valve 76 is wide open and the differential pressure across thisvalve 76 is too low, is the exhaust-gas flap 62 used as a controlvariable of the controller, wherein the exhaust-gas flap 62 is thenadjusted with respect to its pitch, such that the differential pressureacross the LP-EGR valve 76 increases again. The latter is the case inmost operating points. The transition is continuous.

2. Exhaust-Gas Aftertreatment (Driving Operation with RegenerationMeasures):

As exhaust-gas aftertreatment measures it is necessary to regenerate thediesel particulate filter 56 as well as the NO_(x) storage catalyticconverter (NSC) 64. The fresh-air mass 88 is controlled with the help ofthe control flap 36. The exhaust-gas recirculation rate detected throughthe use of the second differential pressure sensor 78 is controlled(second control loop) independently from this.

In this case too, just like before, first the LP-EGR valve 76 is openedand then the exhaust-gas flap 62 is further closed if required. TheNO_(x) storage catalytic converter regeneration is performed in a commonmanner by enriching the fuel air mixture (λ<1) wherein a combination ofa throttling across the exhaust-gas flap 62 and an increased LP-EGR rateis used.

The regeneration of the diesel particulate filter 56 is not achieved byenriching the fuel air mixture but by increased exhaust-gastemperatures, for example by additional injections.

In both cases an optimal operating point with respect to the efficiencyis guaranteed since the LP-EGR valve 76 is always opened first and onlythen is the exhaust-gas flap 62 closed. Thus the throttling losses areminimized.

The HP-EGR path 82 is provided because the cooling effect of thecharge-air cooler 32 during warming up is too large, such that the HCemissions would rise too sharply. This also improves the dynamics,primarily when starting from a vehicle standstill. The exhaust-gasvolume that is present within the entire EGR path (including thecharge-air cooler), must first be replaced by fresh air when driving offfrom a standstill (or in case of a large torque request). A cooling ofthe HP-EGR mass flow 84 is not absolutely necessary; an EGR cooler cannevertheless be optionally provided in the HP-EGR path 82.

The low-pressure exhaust-gas recirculation is especially advantageous inconnection with the NO_(x) storage catalytic converter 64. Thepossibility to regenerate the catalytic converter 64 (DeNO_(x) as wellas DeSO_(x)) is limited by the maximum allowable component temperatures.The limitation is primarily determined by the turbine temperature of theexhaust-gas turbocharger 30. These temperatures can be loweredeffectively during the regeneration with the help of the LP-EGR path 70so that the possible operating range of the catalytic converter isextended towards substantially higher loads when compared to pure HP-EGRsystems.

What is claimed is:
 1. A method for operating a combustion engine, themethod which comprises: providing an internal combustion engineincluding a fresh-air system, an exhaust-gas system, an exhaust-gasturbocharger with a compressor disposed in the fresh-air system and aturbine disposed in the exhaust-gas system; providing an exhaust-gasrecirculation cooler in a low-pressure exhaust-gas recirculation line;providing a pressure sensor in the low-pressure exhaust-gasrecirculation line in flow direction in front of the exhaust-gasrecirculation cooler, the pressure sensor including an absolute pressuresensor; diverting, via the low-pressure exhaust-gas recirculation line,recirculated exhaust-gas from the exhaust-gas system downstream of theturbine of the exhaust-gas turbocharger by using a low-pressureexhaust-gas recirculation valve and feeding recirculated exhaust-gasupstream of the compressor of the exhaust-gas turbocharger as alow-pressure exhaust-gas recirculation mass flow into the fresh-airsystem; providing an exhaust-gas flap in the exhaust-gas systemdownstream from where the low-pressure exhaust-gas recirculation linebranches off from the exhaust-gas system; and opening the exhaust-gasflap and controlling a volume flow through the low-pressure exhaust-gasrecirculation line exclusively with the low-pressure exhaust-gasrecirculation valve as long as a pressure drop across the low-pressureexhaust-gas recirculation valve exceeds a given value and additionallyactuating the exhaust-gas flap in a direction of closing in case thepressure drop across the low-pressure exhaust-gas recirculation valvefalls below the given value in order to control the volume flow throughthe low-pressure exhaust-gas recirculation line.
 2. The method accordingto claim 1, which comprises diverting the recirculated exhaust-gas fromthe exhaust-gas system downstream of a particulate filter.
 3. The methodaccording to claim 1, which comprises diverting, via a high-pressureexhaust-gas recirculation line, recirculated exhaust-gas from theexhaust-gas system upstream of the turbine of the exhaust-gasturbocharger by using a high-pressure exhaust-gas recirculation valveand feeding recirculated exhaust-gas downstream of the compressor of theexhaust-gas turbocharger as a high-pressure exhaust-gas recirculationmass flow into the fresh-air system.
 4. The method according to claim 3,which comprises feeding the high-pressure exhaust-gas recirculation massflow downstream of a charge-air cooler into the fresh-air system.
 5. Themethod according to claim 3, which comprises cooling the high-pressureexhaust-gas recirculation mass flow prior to feeding the high-pressureexhaust-gas recirculation mass flow into the fresh-air system.
 6. Themethod according to claim 1, which comprises measuring a temperature ofthe low-pressure exhaust-gas recirculation mass flow downstream of thelow-pressure exhaust-gas recirculation valve.
 7. The method according toclaim 1, which comprises: calculating a low-pressure exhaust-gas massflow by using absolute pressure ascertained by the absolute pressuresensor; and determining, from the low-pressure exhaust-gas mass flow, avolume flow through a particulate filter disposed in the exhaust-gassystem.
 8. The method according to claim 7, which comprises determining,from the volume flow through the particulate filter, a loading of theparticulate filter.
 9. The method according to claim 1, which comprisescooling the low-pressure exhaust-gas recirculation mass flow prior tofeeding the low-pressure exhaust-gas recirculation mass flow into thefresh-air system.
 10. The method according to claim 1, which comprisesusing, as the internal combustion engine, one of a diesel engine and anotto-cycle engine.
 11. A method for operating a combustion engine, themethod which comprises: providing an internal combustion engineincluding a fresh-air system, an exhaust-gas system, an exhaust-gasturbocharger with a compressor disposed in the fresh-air system and aturbine disposed in the exhaust-gas system and providing an exhaust-gasflap downstream from where a low-pressure exhaust-gas recirculation linebranches off from the exhaust-gas system; diverting, via thelow-pressure exhaust-gas recirculation line, recirculated exhaust-gasfrom the exhaust-gas system downstream of the turbine of the exhaust-gasturbocharger by using a low-pressure exhaust-gas recirculation valve andfeeding recirculated exhaust-gas upstream of the compressor of theexhaust-gas turbocharger as a low-pressure exhaust-gas recirculationmass flow into the fresh-air system; and opening the exhaust-gas flapand controlling a volume flow through the low-pressure exhaust-gasrecirculation line exclusively with the low-pressure exhaust-gasrecirculation valve as long as a pressure drop across the low-pressureexhaust-gas recirculation valve exceeds a given value and additionallyactuating the exhaust-gas flap in a direction of closing in case thepressure drop across the low-pressure exhaust-gas recirculation valvefalls below the given value in order to control the volume flow throughthe low-pressure exhaust-gas recirculation line.
 12. The methodaccording to claim 11, which comprises diverting the recirculatedexhaust-gas from the exhaust-gas system downstream of a particulatefilter.
 13. The method according to claim 11, which comprises diverting,via a high-pressure exhaust-gas recirculation line, recirculatedexhaust-gas from the exhaust-gas system upstream of the turbine of theexhaust-gas turbocharger by using a high-pressure exhaust-gasrecirculation valve and feeding recirculated exhaust-gas downstream ofthe compressor of the exhaust-gas turbocharger as a high-pressureexhaust-gas recirculation mass flow into the fresh-air system.
 14. Themethod according to claim 13, which comprises feeding the high-pressureexhaust-gas recirculation mass flow downstream of a charge-air coolerinto the fresh-air system.
 15. The method according to claim 13, whichcomprises cooling the high-pressure exhaust-gas recirculation mass flowprior to feeding the high-pressure exhaust-gas recirculation mass flowinto the fresh-air system.
 16. The method according to claim 11, whichcomprises: measuring a pressure difference across the low-pressureexhaust-gas recirculation valve; and actuating at least one of thelow-pressure exhaust-gas recirculation valve and the exhaust-gas flap independence of the pressure difference across the low-pressureexhaust-gas recirculation valve.
 17. The method according to claim 11,which comprises measuring a temperature of the low-pressure exhaust-gasrecirculation mass flow downstream of the low-pressure exhaust-gasrecirculation valve.
 18. The method according to claim 11, whichcomprises: calculating a low-pressure exhaust-gas mass flow by using anascertained differential pressure across the low-pressure exhaust-gasrecirculation valve; and determining, from the low-pressure exhaust-gasmass flow, a volume flow through a particulate filter disposed in theexhaust-gas system.
 19. The method according to claim 18, whichcomprises determining, from the volume flow through the particulatefilter, a loading of the particulate filter.
 20. The method according toclaim 11, which comprises cooling the low-pressure exhaust-gasrecirculation mass flow prior to feeding the low-pressure exhaust-gasrecirculation mass flow into the fresh-air system.
 21. The methodaccording to claim 11, which comprises using, as the internal combustionengine, one of a diesel engine and an otto-cycle engine.